1. Field of the Invention
The present invention relates generally to a solid state image pickup device characterized by a charge storage device, and in particular relates to a technique for increasing the dynamic range of a voltage signal generated from a light reception element and for reducing dark current.
2. Description of the Prior Art
A solid state image pickup device has been used as an image pickup element of an electronic camera or a VTR. Such a solid state image pickup device is described in "SOLID STATE IMAGE PICKUP DEVICE", published by Syokodo Publishing Co., Ltd., July 30, 1986. Particularly, the fundamental concept of the solid state image pickup device is described on pages 6 to 13, the photodiode acting as a light reception element and the charge storage action are described on pages 56 to 59 and the arrangement is described on pages 82 to 102.
FIGS. 8 and 9 are a circuit diagram and a block diagram, respectively, showing an example of the conventional solid state image pickup device. A photodiode PD, which is a light reception element, is arranged to discharge a charge corresponding to the quantity of light h.nu.. The quantity of charge discharged from the photodiode PD during a predetermined time period is proportional to the stored quantity of light h.nu. incident on the photodiode PD during the predetermined time period. Although a charge storage capacitor Cs is illustrated as a single independent capacitor, the charge storage capacitor Cs actually comprises a junction capacitance formed together with the photodiode PD to temporarily store the aforementioned charge and to thereby obtain a photoelectrically converted electric signal which is read by a read-out circuit 3 in the succeeding stage.
A MOS transistor Tr1 (hereinafter referred to as a "transistor") performs an ON/OFF operation in response to a Cs reset pulse .phi..sub.RS applied to the gate of the transistor Tr1. As a result, the charge storage capacitor Cs is reset and a reverse bias voltage is applied to the photodiode PD.
When an image is obtained by the conventional solid state image pickup device, it is desirable to improve the obtained picture quality by increasing the dynamic range of the photoelectrically converted electric signal and by reducing dark current noise. However, efforts to increase the dynamic range and to reduce dark current noise have resulted in the following problem.
Specifically, in the conventional solid state image pickup device, a reverse bias voltage is applied to the photodiode PD as described on page 56 of the foregoing "SOLID STATE IMAGE PICKUP DEVICE", and the transistor TR1 is turned on in the circuit of FIG. 8. When a reverse bias voltage is applied, increasing the dynamic range in photoelectric conversion and reducing dark current noise is difficult because the operational range of the photodiode PD is narrow and the generation of a dark current cannot be suppressed.